what do we need to make pong?
1. draw a rectangle and a ball - make variables for all the coordinates (use floats for the ball)
2. move a paddle up and down with the keyboard
3. move the ball - Use variables for the ball speed
4. solve the collision detection for the ball and paddle, and the ball and the walls
5. add the other paddle and solve the 2-buttons at once problem :)
6. what happens if the paddle is moving when the ball hits?
7. keep score
8. go nuts
Here's some code to work from. Because we need to be able to record if two keys are down at the same time I've created a new group of boolean variables. These are like switches, when you press the button it flips the corresponding switch. There is a keyReleased routine that looks to see if the key has been released, and if it has it resets the switch. I have also divvied up the code into a collision routine and a reset routine so it's a little easier to organize.

See if you can look through it and understand what's going on. I know it looks like a lot but break it down and it's not so scary. Use the processing reference page if there is a keyword you do not understand. Or write me if you're confused about something specific an can't figure it out.
joester5 at mac.com

 float ballx = 150;
float bally = 200;
float speedx = 2.7;
float speedy = 0.3;
float player1x = 570;
float player1y = 200;
float player2x = 30;
float player2y = 200;
boolean p1xbutton = false; // booleans are variables that are either true or false.
boolean p1ybutton = false;
boolean p2xbutton = false;
boolean p2ybutton = false;

void setup(){
size(600, 400);
}

void draw (){
background (50);
smooth();
ballx = ballx + speedx; // calculate ball position
bally = bally + speedy;
collision(); // check to see if ball has collided with an edge or paddle
if (keyPressed) {
if (key == 'j'){
p1xbutton = true; // check for button and chandges a boolean array if it has been pressed
}
if (key == 'm'){
p1ybutton = true;
}
if (key == 'a'){
p2xbutton = true;
}
if (key == 'z'){
p2ybutton = true;
}
}
if (p1xbutton == true){
player1y = player1y - 6;
}
if (p1ybutton == true){
player1y = player1y + 6;
}
if (p2xbutton == true){
player2y = player2y - 6;
}
if (p2ybutton == true){
player2y = player2y + 6;
}

rect(player1x, player1y, 10, 50);
rect(player2x, player2y, 10, 50);
ellipse (ballx, bally, 10, 10);
}

void collision(){
if (((ballx > player1x) && (ballx < player1x + 10)) && ((bally > player1y) && (bally < player1y + 50))){ // if it hits player 1 paddle
if (p1xbutton == true){ // if the paddle is going up puts up "spin" on the ball
speedy -= .5 ;
}
if (p1ybutton == true){// down "spin"
speedy += .5;
}
ballx = player1x - 1; // ensures it won't hit twice!
speedx = speedx * -1; // go back
}
if (((ballx < player2x+ 10) && (ballx > player2x)) && ((bally > player2y) && (bally < player2y + 50))){ // if it hits player 2 paddle
if (p2xbutton == true){ // up "spin"
speedy -= .5 ;
}
if (p2ybutton == true){// down "spin"
speedy += .5;
}
ballx = player2x + 11; // ensures it won't hit twice!
speedx = speedx * -1; // go back
}

if (speedy < -4){// keeps the ball from going too far up and down.
speedy = -4;
}
else if (speedy > 4){
speedy = 4;
}

if (ballx > width){
reset(1); // runs the routine reset below and passes the value 1
}
if (bally > height){
speedy = speedy * -1;
}
if (bally < 0){
speedy = speedy * -1;
}
if (ballx < 0){
reset(2); // runs the routine reset below and passes the value 2
}
}

void keyReleased(){
if (key == 'j'){ // check for button released
p1xbutton = false; // changes the boolean to false, letting the computer know that j is no longer being pressed
}
if (key == 'm'){
p1ybutton = false;
}
if (key == 'a'){
p2xbutton = false;
}
if (key == 'z'){
p2ybutton = false;
}
}

void reset(int plyr){
ballx = 300;
bally = 200;
if(random(2) == 2) { // randomly changes the direction when it restarts
speedx = 2.7;
}
else{
speedx = - 2.7;
}
speedy = (random (.6) - .3);
delay (1000); // pauses at the end for a second
}